Vehicles having transmission systems may feature a park lock, which is used to lock the transmission of the vehicle to help to hold the vehicle in a parked configuration. Off road vehicles and the like commonly have a transfer box connected to the transmission via an input drive shaft. The transfer box comprises a differential adapted to transfer motive power from the input drive shaft to the front and/or rear wheels via output drive shafts. A lock can be applied to the transfer box differential so as to restrict the output drive shaft to the front wheels to turn at the same rate as the output drive shaft to the rear wheels.
A driver wishing to park their vehicle might choose to engage the park lock. Failure to apply the vehicle parking brakes or choosing to not apply the parking brakes will normally result in the park lock preventing movement of the vehicle. However, the park lock only prevents movement of the input drive shaft to the transfer box. Thus, in parked conditions where one set of wheels is liable to slip with respect to other set, the transfer box allows movement of one output shaft with respect to the other output shaft even when the park lock is applied. Therefore, it is desirable to lock the transfer box differential when applying park lock but this requires the driver to recognise the conditions and apply each of the park lock and transfer box differential lock.
It is against this background that the invention has been devised. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.